Prior art analog clock recovery circuits typically are slow or, in other words, have a long response time. Further, analog circuitry typically uses considerably more power than digital circuitry performing an identical operation.
Typical prior art digital circuits have required a high sampling rate to obtain appropriate phase discrimination in a clock recovery circuit or phase lock loop type apparatus. The Nyquist sampling theorem requires that the minimum sampling rate be twice the maximum data repetition rate and in practice, a sampling frequency at least eight times the data rate is desirable. If the data being sampled is already of a very high frequency such as 50 megahertz, then a sampling frequency of somewhere in the neighborhood of 400 to 500 megahertz would be required for appropriate phase discrimination using prior art techniques. Such frequencies require the use of very expensive gallium arsenide circuitry rather than the more commonly available CMOS circuits since CMOS circuitry typically has a limitation in state machine environments of approximately 70 megahertz even though the maximum frequency of operation for a single flip-flop could be as high as 250 megahertz.
The present invention allows the use of CMOS while still obtaining appropriate phase resolution in the phase detector by passing a signal through a device such as a delay line to obtain a plurality of signals each having slightly different phases and all of a frequency similar to the repetition rate of the data being detected. This multiple phase set of signals is then logically combined with the incoming data and a tentatively correct recovered clock is generated to obtain information as to the relative phase of the data and the tentative clock whereby a correction signal can be provided to an oscillator that is used to produce this clock in a feedback manner.
It is thus an object of the present invention to provide a digital clock recovery circuit which does not require that the operational frequency capability of the components be substantially higher than the repetition rate of the data being detected.